Here is the batch file I created last night and tested. Although it worked, is my syntax correct? I am using a mapped network drive.
xcopy "Z:\Debby\My Documents\Calibrelibrary\Debs" "Z:\Debby\My Documents\Calibre Saved Libraries\BatchFile BU" /D /S /Y
The syntax looks perfect. Most people forget the quotes around the source & destination paths. It looks like it should only copy the files and folders that have been updated, without prompting you to overwrite existing files. If you remove something in the source, it won't remove it from the backup folder though. If you rename something that has already been backed up, the original AND the renamed file will be in the backup folder after you run the xcopy.
You can add the /V switch to verify that the destination file, once written, can be read. Most of the time this isn't necessary, but since it's a backup the switch is an extra bit of insurance.